1. Field of the Invention
The present invention relates to a rib optical waveguide, various kinds of optical waveguide devices using the same, and an optical waveguide layer device including an optical waveguide layer or film on which a grating, a waveguide lens, etc. are formed and to a method of manufacturing the rib optical waveguide, the optical waveguide devices, and the optical waveguide layer devices by use of liquid materials which are solidified through energy irradiation with an ultraviolet ray and the like.
2. Description of Related Art
A rib optical waveguide includes a substrate, a core layer formed on the substrate, and a rib portion manufactured therein along a light propagation direction to be integral with the core layer. In a plane orthogonal to the light propagation direction, the propagation light is confined along a direction of depth of the core layer in association with a difference between refractive indices respectively of the core layer and the air and a difference between refractive indices respectively of the core layer and the substrate; whereas, the light is confined along a direction of width of the core layer (rib portion) by use of a phenomenon that the effective refractive index is increased in the proximity of the rib portion because the thickness of the rib portion is larger than that of the other core layer portions.
Conventionally, an ion irradiation high-speed etching has been used to manufacture a rib optical waveguide in some cases. However, this manufacturing process has the following problems. The manufacturing process of this method is complex and is not satisfactory in terms of mass production and reproducibility. Owing to the long manufacturing period of time, the production cost is high. Since surfaces of the optical waveguide become coarse through the etching, the dimensional precision is lowered and the light propagation loss is increased. Since etching is employed to fabricate an optical waveguide pattern, the available contour of the optical waveguide (rib section) is restricted.
On the other hand, in the conventional rib-type optical waveguide, since the core layer section in which the rib portion is included is exposed, the waveguide is likely to be damaged, which leads to a problem of inconvenience in handling the waveguide.
In a case of a single-mode rib optical waveguide, when the difference between the refractive indices respectively of the core layer or film and its environmental materials (the air and the substrate) is decreased, the thickness of the core layer and the width of the rib section can be increased, which facilitates the fabrication of the waveguide. However, in the conventional rib optical waveguide, since a portion of the core layer is exposed to the air, when a material having a high refractive index is used to manufacture the core layer, the dimension of the core layer becomes to be smaller and hence the manufacturing thereof is difficult. Moreover, when the difference between the refractive indices respectively of the core layer and the substrate is taken into consideration, there arises a problem that the available substrate materials are limited.
As a branching optical waveguide, there has been known a single-mode Y-shaped branching waveguide manufactured through a thermal diffusion of titanium (Ti) on a substrate of LiNbO.sub.3. This waveguide comprises a basic or fundamental three-dimensional waveguide route and two branching three-dimensional waveguide routes which branch in a Y shape from the basic waveguide route. The difference between refractive indices respectively of the Y-shaped branching waveguide and the substrate is about 10.sup.-3 and the branching angle is about 1.degree. to about 2.degree.. The light passing through the basic waveguide path toward the branching section branches at the branching point to propagate into two Y-shaped branching waveguide routes. Conversely, lights respectively proceeding through the two branching waveguide routes toward the branching section are combined with each other to propagate through the basic waveguide route.
In a Y-shaped branching waveguide of this kind, due to a small difference between the refractive indices respectively of the waveguide portion and the substrate, a large branching angle cannot be obtained. Namely, an increased branching angle leads to a greater radiation loss. This is because the overlapping of the modes in the basic waveguide route and the branching waveguide routes becomes smaller as the branching angle is increased, which lowers the coupling coefficient.
As described above, since a large branching angle is not allowed for the Y-shaped waveguide of the prior art, in order to increase the gap between two branching waveguide paths or routes, it is necessary to elongate the distance of the branching waveguide. Particularly, in a case where Y-shaped branching waveguides are connected in a multiple stage to obtain many branching lights, there is required quite a long distance, which leads to a problem that the device size of the waveguide is increased.
In an optical waveguide produced through a thermal diffusion of titanium (Ti) on a LiNbO.sub.3 substrate, in order to propagate a light through a curved path on the substrate, the waveguide on the substrate is required to have a contour of a curved line. FIG. 115 shows a curved optical waveguide 242 fabricated on a LiNbO.sub.3 substrate 241 through a thermal diffusion of titanium. In such a curved waveguide of the prior art, owing to the symmetric distribution of refractive indices along a direction of the width of the waveguide, the electromagnetic field distribution of the propagation light is expanded toward the outside of the curved portion and hence a radiation loss results. The radiation loss increases as the radius of curvature is decreased in the curve of the optical waveguide. Consequently, in order to minimize the radiation loss, the radius of curvature is required to be increased, which disadvantageously increases the device size of the optical waveguide.
FIGS. 116 shows a state in which a light propagates through the optical waveguide formed through a thermal diffusion of titanium on the LiNbO.sub.3 of FIG. 115. In this figure, a letter r denotes a radial direction of the curve and a curve a denotes a distribution of an effective refractive index n.sub.eff of the optical waveguide, the distribution is symmetric with respect to the central line. In consequence, in the straight-line portion of the optical waveguide, a magnetoelectric field distribution b also becomes symmetric with respect to the central line. However, in the curved portion, a magnetoelectric field distribution c of the propagation light is expanded toward the outside of the curve, which causes a radiation loss as indicated by a hatching portion. For example, in a case of a single-mode Ti-diffusion curved waveguide on a Z-cut LiNbO.sub.3 substrate, assuming that a wavelength of the propagation light is 0.6328 micrometers, a waveguide width is 4 micrometers, a refractive index of the substrate is 2.2, and a difference between the refractive indices respectively of a core and a clad of the waveguide is 10.sup.-3 ; the radiation loss becomes about 3 dB/cm for the radius of curvature to be about 3 cm. In consequence, in order to set the radiation loss to a value not exceeding 3 dB/cm, the radius of curvature is required to be set to several centimeters or more. This leads to a problem of an increase in the size of the waveguide device.
An optical waveguide like the Ti diffusion LiNbO.sub.3 waveguide has an identical contour of the cross section in a longitudinal direction, an identical beam pattern of the propagation light is developed regardless of positions. As above, the conventional optical waveguide has a limited freedom of beam pattern for the guided light; consequently, when an optical coupling takes place with an optical element having a different beam pattern, a radiation loss is caused by the difference between the beam patterns, which lowers the optical coupling efficiency. For example, as shown in FIG. 117, there may occur a case where a light emitted from a laser diode 251 is delivered to a Ti-diffusion optical waveguide 253 formed on an LiNbO.sub.3 substrate 250 such that a light irradiated from the optical waveguide 253 is optically coupled with an optical fiber 252. The light emitted from the laser diode 251 has a beam pattern LBA, which is an elliptic contour with a considerable flatness. In contrast thereto, the light guided through the Ti-diffusion optical waveguide 253 on the LiNbO.sub.3 substrate 250 has a beam pattern, which takes a shape depending on the Ti diffusion state as denoted by letters LBB and LBC. Since the optical fiber 252 includes a core having a circular cross section, the beam LBD of light passing therethrough has a truely round contour. Namely, because the beam pattern of the propagation light varies depending on the optical elements to be optically coupled with each other, a radiation loss occurs in an optical coupling section, which hence leads to a problem of a low coupling efficiency.
As a representative optical channel waveguide, there has been known a Ti-diffusion optical waveguide 261 manufactured by diffusing titanium into a substrate of LiNbO.sub.3 as described above and as shown in FIG. 118. When forming a grating in the optical waveguide 261 of this type, it is difficult or impossible to fabricate the grating only in the optical waveguide portion. Namely, the grating section 262 is inevitably formed in a range wider than the width of the optical waveguide 261.
In consequence, an optical waveguide provided with the grating cannot approach another optical waveguide not having such a grating, which limits the function of the device and which disadvantageously increases the device size.
Furthermore, when fabricating an optical waveguide with a grating section, a process to form the grating is required in addition to an optical waveguide manufacturing process. Consequently, an optical waveguide pattern masking process, a titanium diffusion process, a grating pattern masking process, and an etching process are necessary, which results in a complex manufacturing procedure and which requires a long period of time. As a result, there arise problems that the manufacturing procedure is unsuitable for mass production, that the production cost is increased, and that the reproducibility is deteriorated.
In an optical waveguide like the conventional Ti-diffusion LiNbO.sub.3 optical waveguide, the waveguide is fabricated only on a surface of a substrate. In consequence, there have been problems that the utilization efficiency of the substrate surfaces is not increased, that the waveguide device contour is limited, and that a plurality of substrates or a large-sized substrate is required when many functions are to be included in the waveguide device.
On the other hand, in the conventional optical waveguide, a waveguide functioning as a signal line is arranged in a two-dimensional fashion on a surface of a substrate. Consequently, in a case where a parallel optical signal processing, which is indispensable for realization of an optical computer 1 is to be achieved on optical signals, the amount of information which can be processed at the same time is limited. This leads to a problem that the information processing speed is lowered.
Conventionally, a waveguide grating device is produced as follows. Namely, a glass waveguide is formed on a substrate of silicon (Si) and then a grating device is manufactured on the glass waveguide by means of an electron beam writing apparatus operating under control of a minicomputer.
In consequence, the fabrication process of the grating device is complex and requires a long period of time, and hence is not suitable for the mass production.
An example of manufacturing a plurality of grating couplers on a substrate has been described in the "Waveguide-Type Differential Detection Device For Magnetooptical Disk Pickup" by H. Sunagawa et al. in the material OQE86-177 of Quantum Electronics Group published from the IECE of Japan. The device includes, as shown in FIG. 119 a substrate 270 and three focusing grating couplers 271 to 273 arranged on an optical waveguide of the substrate 270. A light reflected from an optomagnetic disk is irradiated onto these grating couplers 271 to 273 from orthogonal positions just thereover or from inclined positions thereover having an inclination with respect to a line orthogonal to a plane of the couplers. The grating coupler 271 has a grating period, which is slightly larger than grating periods of the other grating couplers 272 and 273. Namely, the grating coupler 271 is formed to excite the light in a transverse magnetic (TM) mode; whereas the grating couplers 272 and 273 are provided to excite the light in a transverse electric (TE) mode. The light reflected from the optomagnetic disk is represented as a composite vector of a P component (Ep) and an S component (Es) of the electric field. The Ep component of the reflection light satisfies a phase matching condition in the grating coupler 271 and hence excites the TM mode light. The Es component of the reflection light meets a phase matching condition in the grating couplers 272 and 273 to excite the TE mode light. The Ep component rarely conducts an optical coupling with the grating couplers 272 and 273, whereas the Es component rarely achieves an optical coupling with the grating coupler 271. As a result, the reflection light from the optomagnetic disk is respectively separated through the grating couplers 271 to 273 into the Ep and Es components so that the lights are focused onto the respective foci P71, P72, and P73. These focused lights are respectively received by optical sensors producing output signals. Based on the output signals, there are generated a disk read signal, a focusing error signal, and a tracking error signal.
In a grating coupler of this kind, the lights of two types of waveguide modes including the TM and TE modes are excited only in the respective separate regions. Namely, the TM and TE mode lights are driven only in the region of the grating coupler 271 and the regions of the grating couplers 272 and 273, respectively. This consequently leads to a problem of a deteriorated light utilization efficiency. In other words, although the light reflected from the optomagnetic disk enters the overall area including the regions of the grating couplers 271 to 273, the optical coupling of the light is accomplished only in the optical waveguide layer, which is only a portion of the area.
The emission (coupling) efficiency of a grating coupler depends on a length of the coupler and an emission loss coefficiency determined by a thickness and a cross-sectional contour of the grating. In consequence, for attaining a highly efficient grating coupler, the length thereof must be increased. This does not necessarily meet the requirement for minimization of the size of optical functional elements.
An optical waveguide lens achieving operations such as a focusing, a divergence, and a collimation on a propagation light passing through a two-dimensional optical waveguide is an essential part when configuring an integrated optical circuit for use in signal processing.
An example of the conventional optical waveguide lens includes, as shown in FIG. 120, a substrate of LiNbO.sub.3, a titanium diffusion waveguide 281 formed thereon, and a grating 282 of a refractive index modulation type for a lens manufactured through a proton exchange.
However, in order to fabricate an optical waveguide lens of this kind, several processes are required, including a process in which a thin titanium layer is accumulated on and is thermally diffused into the LiNbO.sub.3 substrate to form an optical waveguide and a process to create a grating pattern and to achieve a proton exchange for creating a grating section. In consequence, the total number of manufacturing steps is increased and the manufacturing processes becomes complex. Moreover, due to a long period of time necessary for the fabrication, this manufacturing method is not suitable for the mass production.
Furthermore, as an example of the conventional optical waveguide lens, there has been known a geodesic lens. The geodesic lens has a two-dimensional waveguide including a curved portion to form an appropriate curved plane, which serves as a lens for a propagation light. In an example of a geodesic lens formed on an optical waveguide on a substrate, a curved surface to be a geodesic lens is created on a substrate of glass, LiNbO.sub.3, or the like and an optical waveguide is formed on the substrate surface including the curved surface by an ion exchange method.
In order to fabricate a geodesic lens on a substrate, a portion of the substrate is required to be polished in a spherical surface to form a geodesic lens on the substrate. For manufacturing a lens free from aberration, the surface is required to be formed in an aspherical shape. For this purpose, an ultrasonic machining, a diamond honing, a diamond cutting, a diamond polishing, etc. have been employed in the prior art.
However, the curved surface machining methods above are attended with a problem that a long period of work is to be achieved by use of a special apparatus operating with a high precision under control of a computer and hence the methods are not appropriate for mass production.
As an example of an optical element, a grating coupler has been well known. In the grating coupler, a light is guided from an external position into an optical waveguide fabricated on a substrate or a light is emitted therefrom to an external location. Particularly, a grating coupler has been recently adopted in an optical pickup of an optomagnetic disk (reference is to be made to the foregoing material written by Sunagawa et al.) In accordance with the material above, the grating coupler is produced as follows.
On a substrate of Pyrex, a glass #7059 is sputtered and then an Si--N layer is accumulated thereon by use of a plasma chemical vapor deposition (CVD) method. An electron beam resist of a positive type is applied on the resultant surface; thereafter, chromium (Cr) is evaporated thereon to prevent a charge-up phenomenon. By use of an electron beam writing apparatus, a desired grating pattern is drawn on the electron beam resist layer.
After the electron beam writing operation is finished, the chromium film is removed by use of an etching solution and then the electron beam resist is developed with a developer. Moreover, a reactive ion etching (RIE) is accomplished on the Si--N film with the electron beam resist set as a mask. Finally, the electron beam resist is removed by use of a dissolving agent to attain a grating coupler.
The manufacturing method of the prior art above employs electron beam lithography and hence takes a long period of time to create the optical element. This method is also not suitable for mass production. In addition, two etching processes are included and hence the manufacturing processes are required to be controlled with a high precision, which leads to a problem that the production cost of optical elements cannot be reduced.